Most of the bearings used in the wind power generator have an outer diameter of as large as one meter or more, and show the low number of revolutions. This makes the lubricating oil film thinner, so that it has been apprehended that the bearings may soon reach the end of fatigue life. To cope with the problem of fatigue life, the conventional grease compositions employ high-viscosity base oils to ensure a sufficiently thick oil film.
In the general-purpose bearings, the reasons for a short seizure life almost lies in fracture of the oil film due to insufficient supply of the lubricating oil and lack of maintenance of the oil film. As for the bearings used in the wind power generator, however, no special attention has been paid to the above-mentioned problems, and the composition of grease has scarcely become a target of study. This is because the design of the wind power generator is based on the concept that the generator has an oil feeding system therein, which can feed a fresh grease to the portions to be lubricated, thereby keeping the maintenance of the oil film.
However, the conventional grease compositions for bearings used in the wind power generator show low fluidity because the viscosity of the base oil is increased as a measure against the short fatigue life. Therefore, grease cannot be readily replaced at the portions to be lubricated even though the grease is newly fed thereto. As a result, the oil film cannot be repaired successfully, whereby the seizure life of the bearing disadvantageously reaches its end.
Further, the wind power generator is placed outside, and sometimes may be operated under the condition of low temperatures, for example, around −40° C. Unfortunately, the conventional grease compositions for bearings of the wind power generator use high-viscosity base oils, so that there is the problem that the grease compositions cannot be even pumped out for replenishment due to abnormally increased apparent viscosity at low temperatures. Then, the span of the seizure life becomes shorter when the bearing is operated at low temperatures.
For the grease compositions for bearing used in the wind power generator, some measures are taken against the fatigue (flaking) and the wear by addition of additives (JP 2006-161624 A, JP 2007-63423 A, JP 2008-38088 A and JP 2008-111514 A). In those cases, however, shortening of the life span resulting from seizing is not taken into consideration. There is no description about the techniques to improve the fluidity of grease by decreasing the kinematic viscosity of the employed base oil, either.
Recently, the wind power generator has been built on the sea because it is difficult to secure the space for building on the land. When used on the sea, the bearings are exposed to the sea breeze and gather rust by the influence of salt water, thereby causing the problems that the bearings become nonrotatable by the presence of rust, and the rust results in the damage of the bearings. Nevertheless, no consideration has been given to such operating condition where the grease for the bearings is used on the sea and no measure has been taken against rust in the conventional grease compositions.